1. Field of the Invention
The present invention relates to a magnetic reading head including a magneto-resistive effect element.
2. Description of the Related Art
Magnetic recording and reading apparatuses face a demand for an increase in the areal recording density in a scale over 40% every year. Magnetic recording and reading heads provided in these magnetic recording and reading apparatuses also face a demand for higher performances in terms of both of recording and reading performances. Among these heads, it is important for the magnetic reading head to satisfy the following three technical issues of: (1) improvement in higher sensitivity; (2) improvement in the narrowing of a track width; and (3) improvement in the narrowing of a reading gap interval. So far, the demands for higher densities have been responded by means of an anisotropic magneto-resistive effect (AMR) for recording densities ranging from 1 to 10 Gb/in2, or by means of a giant magneto-resistive effect (GMR) that can achieve higher sensitivity for recording densities ranging from 10 to 30 Gb/in2. For recording densities ranging from 20 to 70 Gb/in2, higher areal recording densities have been achieved by means of specular-GMR in which a highly electron-reflective (specular) insulative oxide layer or the like is interposed between boundary surfaces of a GMR structure so as to increase an output with a multiple-reflection effect of electron spin or by means of an advanced GMR effect called a nano-oxide layered GMR (NOL-GMR).
Concerning the magnetic head using the GMR, there are numerous disclosures for a structure called a spin valve as typically reported in Japanese Patent Application Laid-open No. 4(1992)-358310. This magnetic head basically includes three-layered films composed of a magnetic pinned layer, a non-magnetic thin film and a magnetic free layer. Specifically, the magnetic pinned layer is made of a magnetic body, and the magnetization thereof is fixed in a specific direction by use of an antiferromagnetic layer. The magnetic free layer is superposed on the magnetic pinned layer with the non-magnetic thin film interposed in between. The magnetic head includes a magneto-resistive effect element of which electric resistance changes with a relative angle of magnetization defined by the magnetic pinned layer and the magnetic free layer.
The pursuit of higher sensitivity requires even higher reading methods. A GMR method which utilizes an advantage of small element impedance to supply a detection current in a perpendicular direction to a film surface (this method is called CPP-GMR) is considered to be the mainstream for reading from a recording density range from 70 to 150 Gb/in2, for example. A tunneling magneto-resistive effect (TMR) having an extremely high magneto-resistive ratio has an advantage in improving the sensitivity. A conventional basic MR technique using Al2O3 for an insulating barrier layer is disclosed in Japanese Patent Application Laid-open No. 10(1998)-91925 and the like. This film has the maximum magneto-resistive ratio of 70% which is higher than that of the CPP-GMR. However, it is difficult to put this film into practice because the magneto-resistive ratio rapidly drops when the resistance (impedance) of the film is reduced by decreasing the film thickness of the Al2O3 layer.
In a case of CIP-GMR, insulation between an element and a shield is a problem at the time when a shield-to-shield distance is reduced in order to cope with a higher linear recording density. On the other hand, such an insulation characteristic is not an important issue in a case of the CPP-GMR. Moreover, it is considered that problems such as element destruction by heat due to static voltage and current, and an effect of non-linearization due to a magnetic field, are also limited. Although there are numerous reports on the CPP-GMR, a typical example is disclosed in Japanese Patent Application Laid-open No. 7(1995)-221363.
Researches and device developments related to an interaction of spin-polarized currents are increasingly popular in recent years. For example, as disclosed in “Electrical detection of spin precession in a metallic mesoscopic spin valve,” F. J. Jedema et al., NATURE, Vol. 416, pp. 713-316, 18 Apr., 2002, a phenomenon, in which a spin current with spin-polarization is transmitted for a long distance equal to or longer than 100 nm to cause a magnetic interaction, has been actually confirmed. Researchers for this study produced Co strips with mutually different thicknesses and an Al strip located perpendicular to the Co strips, and then formed a structure provided with alumina insulating barrier layers respectively positioned at intersections of each of the Co strips and the Al strip. In this event, when a magnetic field was applied to a film by applying a current from the thicker Co line to the Al line, an electric potential difference due to the magnetic field was generated between another Co line where the current is not applied and the Al line. In this way, a magnetic interaction was confirmed although an interval between the strips exceeded 500 nm. It is theoretically understood in such a form represented by Physical Review B, Vol. 59, No. 1, pp. 93-97 and Physical Review B, Vol. 65, 054401, pp. 1-17, for example, that this magnetic interaction is caused by spin polarized electrons, which are accumulated in interface portions of the AL strip, and which are thus distributed to a wide range of the strips.
In general, in a case where there are two magnetic bodies having different coercivities in an external magnetic field, this magneto-resistive effect element has characteristics that an outputted value of an electric potential of one of the magnetic bodies against a corresponding conductive body changes, and that this electric potential has a different polarity depending on the directions of the magnetization of the two magnetic bodies being parallel or anti-parallel to each other. In the above structure, the magnetic bodies are made of simple Co, and are connected together with Al. A changed output due to a change in the magnetic field is obtained at room temperature by use of this structure.
To achieve a reading sensor having a high output, it is considered to be effective (1) to increase the change in the electric potential from the viewpoint of the material composition, and (2) to amplify the change in the electric potential by means of the structure of the element. Concerning the factor (1), it is important to use a material with high spin-polarization for a material of the magnetic bodies. For a material of the conductive bodies, it is important to use Al, Cu or any other materials having a longer mean free path for the spin electrons than Al and Cu, or to use a material having a function as a d electron conductor. Concerning the factor (2), such a structure that a device is provided with a mechanism for amplifying a voltage signal is disclosed in Japanese Patent Application Laid-open No. 2004-348850.
Usually, it is conceivable that noises in a magnetic sensor include Johnson's noises due to heat, shot noise generated by the electrons tunneling through a barrier, and magnetic noises generated by the magnetization reversal tracked at a high frequency. The Johnson's noises are related to the element resistance, and have small values as well as a small dependency on the frequency. Accordingly, the Johnson's noises are basically common to any device, as white noises.